1. Field of the Invention
This invention relates to a resin sealing type semiconductor device including a resin mold package encapsulating a semiconductor chip, which is mounted on a heat sink and is electrically connected to inner leads extended from a lead frame to define a specific gap with the heat sink through wires.
2. Description of the Related Art
A semiconductor device for power such as a power IC or a driver IC used for an automotive engine control ECU or an automotive ECU for ABS generally includes a semiconductor chip holding a power MOSFET. JP-A-54-18280 and JP-A-6-252315 disclose such kind of a semiconductor device, and the semiconductor device adopts a package holding a heat sink for facilitating heat radiation from the semiconductor chip.
As shown in FIGS. 6, 7, the semiconductor device including the heat sink is conventionally formed using a lead frame. FIG. 6 partially shows the semiconductor device at a step before it is molded into a resin mold package, and FIG. 7 shows a cross-sectional structure of the semiconductor device along a VII--VII line in FIG. 6. In FIGS. 6, 7, after a semiconductor chip 1 is attached (die-bonded) to a heat sink 2 by die-bonding, a pair of opposed sides (only one side is shown) of the heat sink 2 is connected to connection parts 3a protruding from a lead frame 3 by caulking. The lead frame 3 has several inner leads 3b, 3c on both opposed sides thereof. The inner leads 3b, 3c are arranged in two lines so that the front end faces thereof face the both opposed sides of the heat sink 2 with a specific interval therebetween.
The front end faces of the inner leads 3b, 3c are connected to the semiconductor chip 1 through bonding wires 4 by a wire bonding step. In the wire bonding step, as shown in FIG. 8, a wire bonding jig for supporting the heat sink 2 and the inner leads 3b, 3c on the lower side thereof is used. In the conventional structure, because the wire bonding jig 5 is used, the inner leads 3b, 3c are not overlapped with the heat sink 2, and the front end faces of the inner leads 3b, 3c are respectively arranged in a line. Accordingly, there arise a possibility that the lengths of some of the bonding wires 4 exceeds a critical length to connect the front end faces of the inner leads 3b, 3c and the semiconductor chip 1.
To solve the problem, as shown in FIGS. 9, 10, a lead frame structure in which the heat sink and the inner leads are overlapped with one another is proposed. In FIGS. 9, 10, a heat sink 2a to which a semiconductor chip 1 is bonded is connected to a lead frame 6 by caulking. The lead frame 6 has several inner leads 6b, 6c, which are overlapped with the heat sink 2a, i.e, disposed above the heat sink 2a. Some of the inner leads 6b, 6c are perpendicularly bent. Accordingly, the front end faces of the inner leads 6b, 6c face the respective four side faces of the semiconductor chip la with specific intervals therebetween. In this case, the inner leads 6b, 6c and the semiconductor chip la can be connected to one another through the bonding wires 4 having desirable lengths.
In such a lead frame structure, however, the inner leads 6b, 6c are readily deformed in the wire bonding step. This may cause short-circuit between the inner leads 6a, 6b and the heat sink 2a. To solve this problem, for example as shown in FIG. 11, a metallic jig 7 can be inserted into gaps between the inner leads 6b, 6c and the heat sink 2a in the wire bonding step. However, this complicates the wire bonding step. Even if the wire bonding is stably performed without causing the short-circuit, in a molding step which is performed after that, the inner leads 6b, 6c may be deformed due to an injection pressure for mold resin, so that the short-circuit between the inner leads 6b, 6c and the heat sink 2a can occur.